I have a med-large purple tang and have had him for 2 years. 6 inches at purchase, 8 inches right now. He lives with a Regal/pacific blue/Palatte/Hippo (pick your name) tang in a 100 gallon.
They get along fine, but only because Hippo tangs are VERY mellow. Purple tangs are semi-aggressive, and will attack most all other tangs that have the same body shape. I can say that hippo and purple get along fine in a 100 gallon system.
Good Luck, excellent fish, and that signature photo is MY fish.....
David